It doesn't say how much thrust the null article "produced." If there's a significant difference, that's still very suggestive.
My next suspects for error would be that the device is propelling itself through heat dissipation or electrostatic ally by throwing off charged particles. Both of those possible confounding factors are testable, either by measuring heat via IR imaging and computing what propellant effect that heat would have of by dummying up the null device to produce and dissipate an identical amount of energy to the non-null device.
But honestly, I'm a hacker. I'd go rent me a vacuum chamber and put it on a whirlygig and see if it makes it spin. If it spins faster and faster and faster until it flies apart (well okay maybe I'd turn it off before loss of structural integrity), something is definitely off in our understanding of physics and God really will let us have hoverboards. So give me a hoverboard.
if the drive is propelling something to get tens of µN, you could float strips of thin plastic film across the supposed path of reaction matter and it'd be visibly accelerated by collisions with that something, at least if my math is right.
